A roller cone rock bit is a common cutting tool used in oil, gas, and mining fields for breaking through earth formations and shaping well bores. Reference is made to FIG. 1 which illustrates a partially broken away view of a typical roller cone rock bit. FIG. 1 more specifically illustrates one head and cone assembly. The general configuration and operation of such a bit is well known to those skilled in the art.
The head 1 of the bit includes the bearing shaft 2. A cutting cone 3 is rotatably positioned on the bearing shaft 2 which may function as a journal. A body portion of the bit includes an upper threaded portion forming a tool joint connection 4 which facilitates connection of the bit to a drill string (not shown). A lubrication system 6 is included to provide lubricant to, and retain lubricant in, the journal bearing between the cone 3 and the bearing shaft 2. This system 6 has a configuration and operation well known to those skilled in the art.
The bearings used in roller cone rock bits typically employ either rollers as the load carrying element or a journal (as shown in FIG. 1) as the load carrying element. A number of bearing systems are provided in connection with the journal bearing supporting rotation of the cone 3 about the bearing shaft 2. These bearing systems include a first cylindrical friction bearing 10 (also referred to as the main journal bearing), ball bearings 12, second cylindrical friction bearing 14, first radial friction (thrust) bearing 16 and second radial friction (thrust) bearing 18.
FIG. 2 illustrates a partially broken away view of FIG. 1 showing the bearing system in greater detail focused on the area of the main journal bearing. The first cylindrical friction bearing (main journal bearing) 10 is defined by an outer cylindrical surface 20 on the bearing shaft 2 and an inner cylindrical surface 22 of a bushing 24 which has been press fit into the cone 3. This bushing 24 is a ring-shaped structure typically made of beryllium copper, although the use of other materials is known in the art. The ball bearings 12 ride in an annular raceway 26 defined at the interface between the bearing shaft 2 and cone 3.
With reference once again to FIG. 1, the second cylindrical friction bearing 14 is defined by an outer cylindrical surface 30 on the bearing shaft 2 and an inner cylindrical surface 32 on the cone 3. The outer cylindrical surface 30 is inwardly radially offset from the outer cylindrical surface 20. The first radial friction bearing 16 is defined between the first and second cylindrical friction bearings 10 and 12 by a first radial surface 40 on the bearing shaft 2 and a second radial surface 42 on the cone 3. The second radial friction bearing 18 is adjacent the second cylindrical friction bearing 12 at the axis of rotation for the cone and is defined by a third radial surface 50 on the bearing shaft 2 and a fourth radial surface 52 on the cone 3.
Lubricant is provided in the first cylindrical friction bearing 10, second cylindrical friction bearing 14, first radial friction bearing 16 and second radial friction bearing 18 between the opposed cylindrical and radial surfaces using the system 6. It is critical to retain the lubricant in positions between the opposed surfaces of the bearing system. Retention of the lubricant requires that a seal be formed between the bearing system and the external environment of the bit.
Early seals for rock bits were designed with a metallic Belleville spring clad with an elastomer, usually nitrile rubber (NBR). A significant advancement in rock bit seals came when o-ring type seals were introduced (see, Galle, U.S. Pat. No. 3,397,928, the disclosure of which is hereby incorporated by reference). These o-ring seals were composed of nitrile rubber and were circular in cross section. The seal was fitted into a radial gland (see, FIG. 2 and references 60 and 64) formed by cylindrical surfaces between the head and cone bearings, and the annulus formed was smaller than the original dimension as measured as the cross section of the seal. Schumacher (U.S. Pat. No. 3,765,495, the disclosure of which is hereby incorporated by reference) teaches a variation of this seal by elongating the radial dimension which, when compared to the seal disclosed by Galle, required less percentage squeeze to form an effective seal. See, generally, FIG. 2 and references 60 and 64.
Several other minor variations of this sealing concept have been used, each relying on an elastomer seal squeezed radially in a gland formed by cylindrical surfaces between the two bearing elements, and are well known to those skilled in the art. Over time, the rock bit industry has moved from a standard nitrile material for the seal ring, to a highly saturated nitrile elastomer for added stability of properties (thermal resistance, chemical resistance).
The use of a sealing means in rock bit bearings has dramatically increased bearing life in the past fifty years. The longer the seal excludes contamination from the bearing, the longer the life of the bearing and drill bit. The seal is, thus, a critical component of the rock bit.
With continued reference to FIG. 2, an o-ring seal 60 is positioned in a seal gland 64 between cutter cone 3 and the bearing shaft 2 to retain lubricant and exclude external debris. A cylindrical surface seal boss 62 is provided on the bearing shaft. In the illustrated configuration, this surface of the seal boss 62 is outwardly radially offset (by the thickness of the bushing 24) from the outer cylindrical surface 20 of the first friction bearing 10. It will be understood that the seal boss could exhibit no offset with respect to the main journal bearing surface if desired. The annular seal gland 64 is formed in the cone 3. The gland 64 and seal boss 62 align with each other when the cutting cone 3 is rotatably positioned on the bearing shaft. The o-ring seal 60 is compressed between the surface(s) of the gland 64 and the seal boss 62, and functions to retain lubricant in the bearing area around the bearing systems. This seal also prevents materials (drilling mud and debris) in the well bore from entering into the bearing area.
Reference is now made to FIG. 3 which illustrates an alternative implementation for the seal gland retaining an o-ring seal. Like reference numbers refer to same or similar parts. The implementation of FIG. 3 differs from that of FIG. 2 in the implementation of the seal gland geometry.
It is not unusual for the o-ring seal to leak or fail leading to grease starvation in the surface contact zones of the bearing system. This can result in scoring, scuffing, and even catastrophic failure like galling or seizure. It is thus critical to provide a sealing means capable of retaining lubricant in positions of the rock bit between the opposed cylindrical and radial surfaces of the bearing system.
Mechanical (or rigid) face seals have also been developed to effectively separate the mud and debris environment from the lubricant. These seals provide superior performance over elastomer seals in high sliding velocity sealing environments. Such environments subject the elastomer seals to excessive friction and abrasive wear which leads to seal failure.
Reference is made to FIGS. 4A and 4B which illustrate a partially broken away view of a sealing system using a conventional mechanical (or rigid) face seal. The sealing system includes a rigid ring 100 and a resilient energizing ring 102 (for example, in the form of an o-ring). The rings 100 and 102 are installed in a seal gland 104 formed in the roller cone 106. The rigid ring 100 includes a radial seal face 108 which mates with a corresponding radial seal face 110 carried by the cone 106. More specifically, the corresponding radial seal face 110 carried by the cone 106 is formed on a seal sleeve member 112 that is interference fit into the cone 106. The resilient energizing ring 102 is compressed between an inner surface 114 of the rigid ring 100 and an outer sealing surface 116 of the bearing shaft 118. The compression of the resilient energizing ring 102 urges the rigid ring 100 toward the seal sleeve member 112 so as to maintain the radial seal faces 108 and 110 in dynamic sealing engagement and further forms a static sealing surface with the bearing shaft 118.
FIG. 4C shows a plan view of the rigid ring 100 looking towards the radial seal face 108.
It is common in current designs for mechanical face seals used in roller cone rock bits to have extremely high face loads at the radial seal faces 108 and 110 so as to assure sealing contact. This high pressure (for example, on the order of a couple hundred psi, or more) results in a large friction force and high frictional heat flux within the seal. The seal of the rock bit accordingly tends to “run hot,” and thermal-related seal failures (such as ring distortion, excessive wear rate and ring seizure) are commonly encountered.
There is a need for an improved mechanical seal which provides for better thermal performance than prior art mechanical seals. Preferably, a mechanical seal is needed having a configuration which reduces seal operating temperatures and provides for an extended seal life.
Reference is further made to the following prior art references which illustrate numerous examples of mechanical seals for use in roller cone rock bits (the disclosures of all references are incorporated herein by reference): U.S. Pat. Nos. 4,613,004, 4,753,304, 5,570,750, 6,209,185, 6,684,966, 7,128,173 and 7,311,159.